The default cache directory set by oh-my-zsh is $ohMyZsh/cache which
lives in the Nix store in our case. This causes issues with several
completion plugins provided by oh-my-zsh.
Originially, `programs.zsh` sets default values for some
initialisation scripts.
Nix resolves the case of multiple values by concatenating them all.
It is however impossible to predict where the default script will be
inserted; but we never want the default value to override the
user-specified ones.
Now, it doesn't set default values; almost everything is hardcoded at
the begining of the file.
* programs.zsh: add enableOhMyZsh option to automate setup of oh-my-zsh in global zshrc
* programs.zsh: make oh-my-zsh plugins configurable
* programs.zsh: add ohMyZshCustom option
* programs.zsh: add ohMyZshTheme option
* programs.zsh: applying minor fixes to evaluate expressions properly
* programs.zsh: fix ordering of oh-my-zsh config and execution
* programs.zsh: move all oh-my-zsh params into its own scope named programs.zsh.oh-my-zsh
